The long term goal of this research is to understand the development of posture control in infants at risk for significant long-term neurodevelopmental disability. The aim of this pilot study is to examine the development of posture control in high risk infants in order to determine the physiologic basis for neuromotor delays. Specific research questions to be addressed include: 1) Is the coordination of muscle synergies a rate limiting factor in the emergence of posture control in infants with neuromotor delays, or alternatively, 2) Is the ability to adapt postural muscle responses to changing sensory inputs a rate limiting factor in the emergence of posture control? In this pilot study, muscle responses to actual and illusionary challenges to seated balance will be examined in 30 infants aged 8 months (15 high risk and 15 typically developing). Infants will be positioned in a foam seat on a moveable force plate that can be translated back and forth to provide a real challenge to balance. In a second set of experiments, infants will be seated on a non- moving force platform that is surrounded by a "visual box" that can be moved to give infants the illusion of movement. Postural responses to real (platform moving) and illusionary (visual box moving" challenges to balance will be compared between the two groups of infants using surface electromyographic, kinetic (force plate), and kinematic data (digitized videotapes). Results from this preliminary study will improve our understanding of the physiologic basis for neuromotor delays in high risk infants. This information is necessary to improve the specificity of therapeutic interventions designed to reduce the likelihood for persisting neuromotor delays in high risk infants. This information is necessary to improve the specificity of therapeutic interventions designed to reduce the likelihood for persisting neuromotor delays in high risk infants. These results will also allow us, in collaboration with the Experimental Design and Biostatistics Core of RehabNet-West, to develop the most useful measures and to do power analysis that will enhance a subsequent full study.